A major challenge in materials engineering is the controlled assembly of purposefully designed molecules or ensembles of molecules into meso-, micro-, and nanostructures to provide an increasingly precise control at molecular levels over structure, properties and function of materials (Michal, D. W., Nature 2000, 405, 293 and Dai, Z. F., et al., Adv. Mater. 2001, 13, 1339). The controlled synthesis and characterization of low dimensional crystalline objects is also a major objective in modern materials science, physics and chemistry (Polleux, J., et al., Angew. Chem. Int. Ed. 2006, 45, 261 and Angew. Chem. 2005, 118, 267). Many researchers have focused on the rational ways to control the shape, size, and dimensionality of nanomaterials. Self-assembly of inorganic nano building blocks into one-dimensional, two-dimensional, and three-dimensional ordered hierarchical nanostructures are fascinating because the variation of the arrangements of the building blocks provides a method to tune the property of the material (Niederberger, M., et al., J. Am. Chem. Soc. 2002, 124, 13642; Niederberger, M., et al., Chem. Mater. 2002, 14, 4364; Niederberger, M., et al., Angew. Chem. Int. Ed. 2004, 43, 2270; Niederberger, M., et al., J. Am. Chem. Soc. 2004, 126, 9120; Richards, R., et al., Angew. Chem. Int. Ed. 2006, 45, 7277; Richards, R., et al., J. Phys. Chem. C 2007, 111, 12038 and Richards, R., et al., Adv. Mater. 2008, 20, 267).
ZnO is a particularly interesting oxide as an excellent optoelectronic material because of its wide direct band gap and large exciton binding energy. It has been widely studied as catalyst support for methanol synthesis and decomposition from industrial and experimental processes, because methanol can be used as an alternative energy source to diminish oil and gas resource, as well as a raw material for manmade hydrocarbon and their products (BASF, German Patents, 1923, 415, 686, 441, 443, 462, and 837, US Patents, 1923, U.S. Pat. Nos. 1,558,559 and 1,569,755; Sun, Q. et al., J. Catal. 1997, 167, 92 and Olah, G. A. Angew. Chem. Int. Ed. 2005, 44, 2636). ZnO has been proven to be a quite complex and interesting material with a variety of different structures. Therefore many efforts have been exerted to prepare ZnO possessing controlled shapes and morphologies (Yu, H. et al., J. Am. Chem. Soc. 2005, 127, 2378; Wu, J. J., et al., Adv. Mater. 2002, 14, 215; Tian, Z. R., et al., J. Am. Chem. Soc. 2002, 44, 12954 and Zhang, T., et al., J. Am. Chem. Soc. 2006, 128, 10960). Various ZnO structures, such as nanocrystals, nanoparticles, nanocubes, nanowires, and nanosheets have been fabricated successfully. Each of these structures can be formed by a different growth mechanism under a wide range of different thermodynamic conditions.
For instance, ZnO nanostructures have been grown directly from a solid source, such as a Zn foil, or using a ZnO film as a nucleation center for the Zn atoms (Yu, H. et al., J. Am. Chem. Soc. 2005, 127, 2378). ZnO nanowires have also been grown by vapor deposition methods using metal nanoparticles as a catalyst at high and low temperatures (Wu, J. J., et al., Adv. Mater. 2002, 14, 215). Large arrays of oriented helical ZnO nanorods and columns were formed using simple citrate ions to control the growth behavior of the crystal (Tian, Z. R., et al., J. Am. Chem. Soc. 2002, 44, 12954). Complex and oriented ZnO nanostructures were synthesized by taking advantage of the preferential adsorption of organic structure-directing agents on different facets of hexagonal ZnO crystals (Zhang, T., et al., J. Am. Chem. Soc. 2006, 128, 10960 and Tian Z. R., et al., Nature Mater. 2003, 2, 821). The interests in fabricating new ZnO nanostructures have been steadily growing due largely to the exciting new applications (Huang, M. H., et al., Science 2001, 292, 1897 and Wang, Z. L., et al., Science 2006, 312, 242), which imply the importance of controlling size and shape in ZnO synthesis.
Perfectly ordered oxide surfaces are usually quite inert, so that their chemical and catalytic properties are commonly attributed to the presence of surface defects (Kovacik R., et al., Angew. Chem. Int. Ed. 2007, 46, 4894). ZnO is widely used in catalysis, electrical devices, optoelectronics and pharmaceuticals, which often crucially depend on the defect properties of this versatile material. It is becoming increasingly established that in order to control the functional properties of nanoscale materials, it is necessary to control not only their composition, shape and size, but also their defect structure (Spence J. C. H., Science 2003, 299, 839). To understand and to control the defect content of inorganic nanostructures can be seen as an important goal (lschenko V., et al., Adv. Funct. Mater. 2005, 15, 1945). However, there is little research about the direct fabrication of ZnO structures which has rich defects, though many efforts have been exerted to prepare ZnO possessing controlled shape and size.
Benzyl alcohol has been found to be a successful medium to tailor metal oxides with well-controlled shape, size and crystallinity under anhydrous conditions, for example, TiO2 nanoparticles of anatase phase in the 4-8 nm size range (Niederberger, M., et al., Chem. Mate. 2002, 14, 4364-4370). Vanadium oxide nanorods and tungsten oxide nanoplatelets with identical morphology (Niederberger, M. et al., J. Am. Chem. So. 2002, 124, 13642) were synthesized in this medium by Stucky and co-workers from metal chloride precursors. Bimetallic oxides of Perovskite structured BaTiO3, BaZrO3, LiNbO3 (Niederberger, M., et al., Angew. Chem. In. E. 2004, 43, 2270) and SrTiO3, (Ba, Sr) TiO3 nanoparticles (Niederberger, M., et al., J. Am. Chem. So. 2004, 126, 9120) with controlled particle size and high crystallinity have also been prepared through a suggested C—C bond formation mechanism using metal alkoxides as the starting materials. In all of these studies, no selectivity in surface growth and no nanoscale building rods with rich holes were found. A general drawback of the above sol-gel processes employing benzyl alcohol for tailoring metal oxides with well-controlled shape, size and crystallinity, is the amorphous nature of the derived materials, and the following heat treatment to induce crystallization which usually leads to undesired particle morphology.